Heat treated steel article



United States Patent 3,.i17,%1 HEAT TREATEB STEEL ARTKILE Donald P.Koistinen, Birmingham, Mich, .assignor to General Motors Corporation,Detroit, Mich a corporation of Delaware No Drawing. Fiied June 21, 1960,er. N 37,583 2 (Iiaiins. ({Il. 14S39) This invention relates to the heattreatment of metals and, more particularly to a new method of making athrough-hardened steel part which has a fatigue life superior tothrough-hardened steel parts made by conventional heat treating methods.

The present invention is a continuation-in-part application of mypreviously filed co-pending patent application Serial No. 674,038, whichwas filed July 25, 1957, and now abandoned.

It is well known that the fatigue life of a steel part can besubstantially increased by imposing a favorable residual stress on thesurface of the part. Ordinarily a favorable residual stress is one whichis opposite to that imposed on the part during use, a residualcompressive stress being most generally used to improve the fatigue lifeof a part subjected to tensile stresses. Heretofore a residualcompressive stress layer could only be formed on the surface of athrough-hardened steel part by a special mechanical treatment performedon the part subsequent to the heat treating or hardening operation.Treatments, such as rolling, shot peening, sand blasting and severegrinding, are typical of the methods which have been employed; however,each of these methods entails a separate mechanical process subsequentto heat hardening in order to accomplish the desired results.

In particular, it is an object of the present invention to provide athrough-hardened steel part having a substantially thicker surface Zonewhich is in a state of residual compressive stress than that formed withprior mechanical methods.

This object and other objects, features and advantages of the presentinvention will be more apparent from the following description ofpreferred examples thereof.

Briefly, e invention encompasses subjecting a throughhardening steelpart to a novel heat treatment in which, during quenching, the surfaceof the part is hardened subsequent to its interior. This, of course, isdirectly opposite to the effect which obtains during a conventional heattreatment. In the invention an alloying element is preferentiallydissolved at a high temperature in a selected area of athrough-hardening steel part. The part is then quenched to obtain asequential hardening of the part in which the selected area hardensafter the balance of the part. A residual compressive stress is inducedon the surface of a part by preferentially dissolving an alloyingelement at the surface of the part and quenching the part tosuccessively harden the interior and then the exterior.

The invention is more aptly described in connection with the efiects itproduces on a through-hardening steel. For this reason reference isherewith made to the nature of a through-hardening steel and the mannerin which it is affected by a hardening heat treatment. A heat treatmentis generally understood to be a combination of heating and coolingoperations, timed and applied to a metal or alloy in the solid state ina way that will produce desired properties.

In general terms, a through-hardening steel is a steel which contains asufiicient amount of carbon in its composition (generally about 0.5% ormore) to produce a martensite transformation when the steel isconventionally quenched from an austenitizing temperature. For purposesof this invention, by the term through-hardening steel I mean toencompass only those steels which illlfiil Patented Jan. 7, 19%!- willundergo a martensite transformation substantially throughout the crosssection of a part formed of that steel. Accordingly, if a steel part canbe throughhardened, my novel heat treatment can be effectively used.

A through-hardening steel is normally hmdened by a heat treatment inwhich the steel is heated for a sufficient time at a temperature highenough to render the steel soft and ductile. The temperature at whichthe steel has been heated to make it soft and ductile is called theaustenitizing temperature. Rapidly cooling the hot austenitized steel,such as by quenching it in water, brine, oil, etc., causes a hardeningof the steel to occur. Austenitiz-ing, as is well known in themetallurgical art, is the process of forming austenite by heating aferrous alloy into the transformation range (partial austenitizing) orabove the transformation range (complete austenitizing). Thetransformation range referred to is the range in which alpha irontransforms into gamma iron or, more generally, the range in whichferrite is converted to austenite.

in the austenitizing process, then, iron is transformed from the alphaphase (a body-centered cubic crystalline structure) to the gamma phase(a face-centered cubic crystalline structure). Ferrite is a solidsolution in which the alpha iron is the solvent, while austenite is asolid solution in which gamma iron is the solvent. Cementite, on theother hand, is a compound of iron and carbon which may co-exist undersome conditions with either the ferrite or austenite. However, thehigher the austenitizing temperature, the greater the amount ofcemen-tite dissolved in the austenite. The A temperature is thattemperature at which austenite begins to form during heating and thus isthe lowest temperature at which cementite can be dissolved in autenite.The A temperature of la hypereutectoid plain carbon steel is thattemperature at which the solution of cementite in austenite is completedduring heating.

The sudden cooling of austenite changes the crystalline structure fromthe soft and ductible gamma form to the hard and brittle tetragonalcrystalline form called martensite.

Austenite, the high-temperature gamma phase in steel, decomposes duringcooling into such products as ferrite, cementite, pearlite, bainite ormartensite, depending upon the cooling conditions and the composition ofthe steel. The individual characteristics of these products control theproperties of a steel. As the method of cooling governs the formation ofthese products, the method of cooling also governs the chartacteristicsof the resultant steel article formed.

In the usual hardening of steel, the desired transformation product ismartensite, and the cooling rate must be sufficiently rapid to preventaustenite from decomposing into any of the other possible products, suchas ferrite, cementite, pearlite and 'bainite. Fortunately, time is amajor factor in the formation of these other products, and theirformation can be suppressed by a sufficiently rapid cooling through thetemperature ranges at which they characteristically form. Thus it ispossible to supercool the austenite to the particular range wheremartensite is formed without incurring substantially any decompositionof the austenite into the other crystalline forms.

be regulated. By establishing the desired M relationship between theinterior and surface regions of a part, the interior can be expandedbefore the exterior. By thus regulating the relative time and locationof expansion, a predetermined stress distribution can be obtained duringthe hardening.

The M temperature is dependent upon the proportion of alloying elementwhich is actually dissolved in the austenite rather than that which istotally present; the greater the proportion of the alloying elementdissolved, the lower the M temperature. As previously indicated inconnection with cementite, the proportion of the alloying element(carbon) dissolved in the austenite can be increased by heating.Hypereutectoid plain carbon steels, those having a carbon contentgreater than about 0.8% by weight, as well as certain hypereutectoidalloy steels, possess the unique characteristic of having a variable Mtemperature. The M temperature of these steels is predominantlydependent upon the temperature at which the steel was austenitizedimmediately prior to quenching, the M temperature being an inversefunction of the austenitizing temperature. Some alloy steels areoppositely affected, and an increase in the austenitizing temperaturecauses an increase in the M temperature. Although the concepts of theinvention might be used in connection with the latter type steel, it isthe former type of steel which readily lends itself to the practice ofthe invention.

In its most useful application, the invention involves forming acompressive stress on the surface of a throughhardening steel part. Thisresult is attained by establishing a suitable M relationship in whichthe M temperature of the interior is higher than the M temperature ofthe exterior, causing a sequential transformation of first the interiorand then the exterior into martensite when the part is quenched.

The first modification of the invention is directed to forming thepeculiar M relationship by means of a twostep austenitizing treatment.The part is initially through-heated at one austenitizing temperatureand then heated for only a short time at a second aus-tenitizingtemperature. A hypereutectoid plain carbon steel or alloy steel, such asone containing nickel or chromium, is first heated throughout at acomparatively low austenitizing temperature. Then, before substantiallyany cooling, the steel is heated for only a short period of time at acomparatively high austenitizing temperature, the duration beingsuflicient to heat only a surface zone of the part to the highertemperature.

A typical example of an alloy steel having this unusual property is SAE52100. This steel is approximately composed as follows: 0.95 %l.l0%carbon; 0.25%- 0.45% manganese; 0.025% maximum phosphorus; 0.025%maximum sulphur; 0.20%0.35% silicon; 1.30%1.60% chromium; allproportions by Weight, and the balance iron.

The following example, employing SAE 52100 alloy steel, will serve toillustrate the variable M temperature of chromium alloy steels. When anSAE 52100 alloy steel (typically 1% carbon and 1.5% chromium) is heatedat an austenitizing temperature of about 1450" F. for a suificientduration of time, it becomes soft and ductile. Upon quenching the hotalloy, it is found that the M temperature is approximately 500 F. Bysimilarly austenitizing the same steel at about 1900" F., the Mtemperature, upon quenching, is found to be approximately 250 F. Thislowering effect on the M temperature through austenitization at anincreased temperature is more particularly evidenced in steelscontaining an increased proportion of chromium. The effect is more pronounced in chromium steels containing more than about 1% by weightchromium.

By suitably quenching a part having two M temperatures, for example, inoil, so that there are no large temperature gradients formed duringquenching, the interior of the part will reach its M temperature andstart to transform into the hard and brittle martensite before thesurface zone reaches its respective M temperature. In this sequence oftransformation, the surface zone, which is still austenite and hencequite soft and ductile, readily acconnnodates the expansion of theinterior as it transforms into mar-tensite. As the surface yields byplastic deformation to the expanding interior, no residual stressresults in the interior, as opposed to the residual compressive stresswhich normally results therein when expansion occurs against a hardenedexterior.

However, when the surface zone transforms into the martensite structure,the accompanying expansion is not so readily accommodated by the alreadyhardened interior. There is no plastic deformation of the interiorregions since the interior has already transformed into the hard brittlemartensite structure. Thus, the expansion of the surface during theaustenite-martensite transformation results in imposing a residualcompressive stress on the surface of the hardened steel part.

The depth of the residual compressive stress layer at the surface can bereadily adjusted by varying the thickness of the surface layer which hasbeen heated to the higher austenitizing temperature.

Since the surface portions ordinarily cool more rapidly than theunderlying regions, the quenching rate should be comparatively slow,such as that afforded by an oil quench, so that the smallest possibletemperature gradient can be maintained between the inner core and thesurface. The small gradient is employed to permit the interior of thepart to reach its M temperature and to substantially transform into themartensite structure before the surface layer reaches its M temperatureand is hardened.

A residual compressive stress can be imposed in this manner on thesurface of a metal part, such as a bearing ball, which is made of thechromium alloy steel SAE 521-00. The bearing ball is heated for asuflicient duration of time to austenitize it completely throughout at alow, austenitizing temperature of approximately 1450" F. The bearingball is then immediately subjected to the increased temperature ofapproximately 1900 F. for a short period of time in order toaustenit-ize only a surface portron thereof at the increasedtemperature. The duration of the second heating is determined primarilyby the physical dimensions of the part and can vary upwardly from afraction of a second, such as required for the second heating of smallbearing balls.

After the two-step heat treatment the bearing ball is rapidly cooled byquenching in oil. The interior, having an M temperature of approximately500 F. due to the lower austenitizing temperature, transforms into themartensite structure first. The accompanying expansion against thereadily deformable outer surface imposes no residual stress on theinterior. The subsequent expansive transformation of the outer layer ofthe ball at approximately 250 F., due to the higher austenitizingtemperature, creates a resulting compressive stress thereon.

The oil quench is satisfactory for cooling smaller parts wherein thetemperature gradient between the interior and surface portions can bemaintained fairly small. For larger articles, an oil quench may prove tobe unsatisfactory for maintaining the small gradient, and in suchinstances it is preferable to use a two-step quench. A two-step quenchpreferably involves initially immersing the hotpart in a molten saltbath for the first step, the temperature thereof being between the two Mtemperatures. Water, oil, brine, etc., can be used for the second stepof the quench. In this manner, the interior of relatively large articlescan be substantially transformed into martensite structure before theexterior reaches its M temperature.

Although the austeni-tizing temperature is particularly effective indetermining the M temperature of chromium allo steels, the method ofthis invention is not specifically limited thereto. This method can beeffectively employed to impart a compressive stress on the surface ofany steel which exhibits a sufiiciently rapid lowering of its Mtemperature during the second heating step. The lowering of the Mtemperature must be sufficiently rapid so that the effect of thetemperature gradient between the interior and surface portions is notdestroyed.

Hypereutectoid plain carbon steels, for example, will exhibit a lower Mtemperature with an increased austenitizing temperature between the Aand the A temperatures. A hypereutec-toid plain carbon steel ispreferably initially austenitized completely throughout at a temperatureonly slightly above the A temperature. The second comparatively shortheating would preferably be at about the A temperature to obtain themaximum differential between the resultant M temperatures. The specifictemperatures employed, duration of heating, type of quench, etc., ofcourse, are variable; optimums for each type of steel, part size andconfiguration will vary.

A second modification of the invention involves preferentiallydissolving an alloying element in a surface zone of a part using asingle austenitizing temperature rather than the dual-temperatureaustenitizing treatment previously described. The second modification ofthe invention produces the particular M differential desired toestablish a predetermined stress distribution by introducing a selectedalloying element into the surface zone of the part from the environmentused during austenitizing. In this modification, a part is concurrentlyaustenitized while the alloying element is diffused from theaustenitizing environment into the surface of the part. To impart acompressive stress at the surface of a part, a suitable alloying elementis diffused into the surface, causing that area to have a lower Mtemperature than the balance of the part.

I have found that by austenitizing a through-hardening steel in thepresence of a nitrogen-containing gas, such as ammonia, nitrogendiffuses into the surface of the part while it is concurrently beingaustenitized. When nitrogenizing a through-hardening steel part in thismanner, nitrogen diffuses into the surface of the part and is dissolvedin the solid solution of austenite, causing a reduction in the Mtemperature of that area containing the dissolved nitrogen. At theaustenitizing temperature, the nitrogen dissolves, as opposed to formingthe hard iron nitride particles common to conventional nitriding.

Bearing balls having a diameter up to about one-half inch, and races forthese balls have been treated in accordance with this modification ofthe invention. Balls made of SAE 51100 alloy steel and races made of SAE52100 alloy steel were similarly treated. The bearing element wasinitially subjected to an austenitizing temperature of approximately1575 F. for approximately thirty minutes in a slightly reducingatmosphere containing approximately 5% ammonia. The bearing element wasthen quenched in oil from the austenitizing temperature to roomtemperature. It was thereafter further quenched to a temperature ofapproximately c F. to reduce the proportion of retained austenitepresent. Tempering does not destroy the results of the invention, andthe elements are preferably tempered for approximately one hour at atemperature of about 300 F. The bearing elements were found to have asurface zone approximately 0.010 inch thick that was in a state ofcompression.

A deeper surface layer was imposed by using the following treatment. SAE51100 and 521 00 alloy steel bearing elements were austenitized for anhour at about 1575 F. in the above-described ammonia-containingatmosphere and oil-quenched to room temperature. These parts were thenre-austenitized at about 1575 F. in the same ammonia-containingatmosphere for approximately thirty minutes, oil quenched to roomtemperature, quenched to about 100 F., and fiinally tempered for aboutan hour at about 300 F. The re-austenitizing produces a finer grain sizeand may therefore be a preferred process in certain instances. Thebearing elements resulting had a compressively stressed surface layer ofabout 0.015 inch in depth.

This modification of the invention is especially useful in that itprovides a highly consistent means of forming a specific predeterminedstress distribution under commercial production conditions. It is to beunderstood that any alloying element which will adequately reduce the Mtemperature and which can be analogously diffused to a sufficient depthcan be used. Austenitizing in an atmosphere containing chromic chloride,nickel, carbonyl, etc., may provide satisfactory results in certaininstances. However, this modification of the invention can be mosteffectively commercially practiced employing nitrogen. Nitrogenradically affects the M temperature of a through-hardening steel andwill diffuse quite deeply into the surface of such a steel part.

The invention contemplates still a third means of establishing thepeculiar M relationship which is characteristic of the invention. Thethird modification of the invention encompasses initially applying analloying element to a selected area of the part before it is heattreated. The part is then austenitized, at which time the previouslyapplied alloying element is dissolved in the austenite of a selectedarea of the part. The dissolving of the alloying element in the selectedarea, as hereinbefore mentioned, produces a lowering the M temperatureof that area.

More specifically, this modification of the invention concerns initiallytreating a through-hardening steel part to apply the alloying element tothe surface of the part. The alloying element may be applied as acoating on the surface, or the surface may be impregnated with thealloying element. An alloying element, such as chromium or nickel, couldbe electro-deposited onto the surface of the part and subsequentlydiffused into the surface during the austenitizing heat treatment. inthis manner, only a single austenitizing temperature need be used sinceonly a surface zone of the part would be affected by the previouslyapplied alloying element. The duration of the austenitizing treatmentwould determine, in part, the depth to which the alloying element woulddiffuse and, consequently, the deph of the surface zone which would beaffected; hence, the zone put under compression when the part issubequently quenched.

The surface of a through-hardening steel part is impregnated with an Maffecting alloying elment when it is conventionally nitrided, such asimmersion in a fused salt bath or the like. In my method, the discreetparticles of iron nitride formed in the conventional nitr-iding would bedissolved during the subsequent austenitizing treatment and produceeffects analogous to those obtained in accordance with the secondmodification of the invention. A through-hardening steel could also beboronized by electrolysis in fused borax prior to heat treatment thereofin my manner. The effects produced are analogous to those obtained byconventionally nitriding such a steel prior to heat treatment thereof inmy manner.

-A rolling contact bearing element formed of SAE 51100 or SAE 52100 canbe treated in the following manner. The element is treated for fivehours at about 975 F. in approximately a 15%25% dissociated ammoniaatmosphere, and immediately thereafter at about 1050 F. for anadditional five hours in approximately a 83%- 86% dissociated ammoniaatmosphere. With or without an intermediate cooling, the elements aresubsequently austenitized at about 1550" F. for about twenty minutes,quenched to room temperature, then quenched to about F. in a DryIce-acetone mixture. It is then tempered for about thirty minutes at 300F. The resulting element made in this manner exhibits a compressivelystressed zone extending to a depth of about 0.045 inch.

The above-mentioned second and third modifications of the inventionprovide the most suitable commercial production means of attaining thebenefits of the invention. The compressively stressed surface layer canbe formed so deeply With the invention that even the usual surfacegrinding used in finishing rolling contact bearing elements only removesa small portion of the compressively stressed surface region. Bearingballs up to about one-half inch can be made having compressivelystressed surface regions greater than about 001 inch and race elementsgreater than about 0.005 inch.

It is to be understood that although the invention has been described inconnection with certain specific examples therefor, it is not intendedthat the invention be limited thereby, except as defined in the appendedclaims.

I claim:

1. A finished rolling contact bearing rolling element having a diameterup to about one-half inch, said element being formed of athrough-hardening steel, said element being hardened to generally thesame hardness substantially throughout its cross section, and acompressively stressed surface zone on said element extending to a depthgreater than about 0.010 inch.

2. A finished rolling contact bearing race element formed of athrough-hardening steel, said element being hardened to generally thesame hardness substantially throughout its cross section, and aeornpressively stressed surface zone on said element extending to adepth greater than about 0.005 inch.

References Cited in the file of this patent UNITED STATES PATENTS1,813,507 Ramage July 7, 1931 1,987,841 Rittershausen Ian. 15, 19352,086,801 Hayden July 13, 1937 2,378,300 Hodge June 12, 1945 2,437,249Floe Mar. 9, 1948 2,470,988 Kanter May 24, 1949 2,493,951 DruyuesteynIan. 10, 1950 2,779,697 Chenault et a1. Jan. 29, 1957 2,843,374B-oegehold July 15, 1958 2,875,112 Dusseldorf et al Feb. 24, 19592,921,877 Samuel et al. Jan. 19, 1960 OTHER REFERENCES The Iron Age,Steel Shot Speeds, Feb. 2, 1950, pp. 82-85. (Copy in Div. 3 of PatentOfiice.)

1. A FINISHED ROLLING CONTACT BEARING ROLLING ELEMENT HAVING A DIAMETERUP TO ABOUT ONE-HALF INCH, SAID ELEMENT BEING FORMED OF ATHROUGH-HARDENING STEEL, SAID ELEMENT BEING HARDENED TO GENERALLY THESAME HARDNESS SUBSTANTIALLY THROUGHOUT ITS CROSS SECTION, AND ACOMPRESSIVELY STRESSED SURFACE ZONE ON SAID ELEMENT EXTENDING TO A DEPTHGREATER THAN ABOUT 0.010 INCH.